Process for direct nickel-plating of aluminum and its alloys



Patented Mar. 4, 1941 PATENT OFFICE PROCESS FOR DIRECT NICKEL-PLATING OF ALUltIINUM AND ITS ALLOYS Jean Frasch, Paris, France, assignor to Societe de Produits Chimiques des Terres Bares, Paris,

France No Drawing. Application June 28, 1937, Serial 6 Claims.

The object of the present invention is a process for'direct nickel-plating of aluminum and its alloys, including those aluminum alloys which contain a high proportion of magnesium.

Many attempts have been made for nickelplating aluminum which include the previous reduction of the thin film of oxide which is usually on the metal, such methods often include a preliminary coating with iron, zinc, copper or brass, such coating being obtained either by chemical transfer or by electrolysis. However, the methods heretofore known have not usually given satisfactory results in their industrial application, since the alumina film is once more formed if the reduction is not instantaneously followed-by the first chemical or electrolytical coating.

The previous attack of aluminum by chlorides, which dissolve the alumina thus formed have generally not given satisfactory results owing to the absorption of chlorine compounds in the porosity of the metal, in which such chlorine will remain even after the subsequent electrolytical deposition. Even if the nickel-plating is initially quite adherent, such absorbed chloride is liable to cause a separation of the nickel plating, after a time. and especially when the temperature is raised.

The greatest destroyer of aluminum and of its alloys in connection with electro-plating is always the thin film of alumina which forms after degreasing the aluminum article in an ordinary degreasing bath of sodium hydroxide, sodium cyanide or even sodium carbonate or phosphate, thus requiring (owing to the necessity of degreasing before the electrolytic treatment) the use of a special degreasing bath for the said metal.

On the other hand the reduced affinity of aluminum towards the metals which have been tried for forming an intermediate layer, such as copper, iron, brass which are themselves easily nickel-plated, have induced the workers in this art to reject such solution which does not aiford satisfactory results.

The object of the present invention is a direct nickel-plating process for aluminum and its alloys which can overcome the above mentioned disadvantages.

According to the invention, I use for the degreasing treatment an electrolytic bath containing besides the caustic base, the cyanide and other salts usually employed for degreasing, a sulphate of a metal which is between aluminum and nickel in the electromotive iorce series, 1. e. a sulphate of a metal that is less electropositive than aluminum, but more electropositive than nickel, and to In France July 2, 1936 which bath are preferably added organic colloidal substances. As sulphates I use for instance a sulphate of a heavy metal, such as manganese, zinc, cadmium. The said sulphate will be used in proportions by weight which are substantially equal to the alkaline cyanide contained in the bath.

At the same instant at which a particle of nascent hydrogen formed by the electro-chemical dissociation of the caustic base or of its salts removes a particle of grease or reduces a particle of oxide, a thin deposit of metal is formed and eventually also of organic colloidal substances which prevent the formation again of alumina upon the removal from the degreasing bath.

n the other hand, according to the present invention, I prefer to adjust the electrolytic current circulating through the bath at a voltage which is substantially lower than the voltages usually applied in electrolytic degreasing baths. Consequently, instead of applying a voltage of 8 to volts (or even 6 volts7, I maintain such voltage below 2 volts and preferably at 1 to 1.5 volts, in order to slow down the degreasing and reduction treatments; the simultaneous deposit takes place more regularly and uniformly. Notwithstanding the said low voltage, the degreasing operation is completed by /2 to 1 minute and the piece is ready for nickel-plating.

It should be noted that the presence of ion S04 is necessary since it acts as catalyst in the degreasing reactions.

After being degreased, the piece is rinsed in running water and then passed through the nickel-plating bath.

According to the invention, the said bath is a bath of polyaminonickelo-sulphate, i. e. of a form such as NiSOANI-ma in which one or more of the (NHa) groups may be replaced by the corresponding number of (H), such as for instance the:

hexa-amino-nickelo-sulphate NISOANH-r) e or the:

di aqua tetra NiSO4(NH3)4(H2O)2.

I preferably add ammonium acetate to the bath to maintain thestability of the said nickel compounds, and add a magnesium salt (e. g. sulphate or chloride, for increasing the conductivity of the bath and for increasing the malleabllity of the deposited nickel. The said bath may be cold or hot operated under 2 to 4 volts and .25 to 6 amperes per square decimeter.

In order to avoid the objectionable effect of its variations I maintain constant the pH value of amino nickelo sulphate such bath at the predetermined value, either by -may be advantageously used for the degreasing bath and for the nickel-plating bath:

- 1. Dsdmsmo Per cent Manganese sulphate 8 to 5 Sodium cy 2 to 8 Ammonia 2 to 8 Gela n .5 to 1 Zinc sulphate; 4 to 8 Sodium cyanide '4 to 8 Caustic potash 2 to 8 Dextrin. .5 to 1 2. NICKEL-PLATING v Per cent Hexa-annno-nlcldelo-sulphate 15 to 45 Ammonium acetate 1 to 5 Magnesium sulphate 10 to 15 Di-aqua-tetra-amino-nickelo-sulphate- 15 to 45 Ammonium sulphate 2 to 8 Magnesium chloride 8 to 10 methods and particularly the nickel structure, as

well as the pieces provided with such coatings.

I claim: 1. A process of nickel-plating metallic objects andinwhichthesaid compound is hexa-amino-nickelo-sulphate.

of aluminum or its alloys, which comprises first subjecting said objects to an electrolytic degreesing treatment in an alkaline bath containing several per cent oi a soluble cyanide and contain? ing several per cent oi a sulphate of a metal more electropositive than nickel and less electropositive than aluminum, said metallic object constituting the cathode in such electrolytic treatment and subsequently electrolytically depositing nickel thereupon.

2. A process as covered in claim 1, in which the V degreasing bath also contains an organic colloid, treatment is eiiected under a voltage-which volts.

3. A process oi! nickel-plating aluminum objects and aluminum alloy objects which comprises first electrolytically degreasing said objects in a bath containing several per cent or a cyanide,

several per cent oi an alkali and several per cent of a sulphate or a metal more electropositive than nickel and less electropositive than aluminum, and then electrolytically depositing nickel thereupon, while said obl'ect constitutes the cathode in an electrolytic bath which contains dissolved polyamino-nickelo-sulphate.

4. A process according to claim 1 in which the nickel-plating bath comprises a polyaminonickelo-sulphate and a substance selected from the group consisting of ammonium acetateand magnesium sulphate.

5. A process of nickel-plating a metallic object which contains metallic alumium asa major com. ponent, which comprises first degreasing said object while it constitutes the cathode in an electrolyte which contains several per cent of an alkali and several per cent of cyanide and several per cent of zinc sulphate, and thereafter electroplating nickel thereupon while said object constitutes the cathode in an electrolyte which is an aqueous solution of polyamino-nlckelo-sulphate, ammonium acetate and um sulphate.

6. A process as in claim 5,'in which the nickel JEAN FRASCH.

less than 2. 

